Modern manufacturing processes and quality assurance methods often require very accurate measurements of surfaces with respect to their planarity, profile or roughness. Preferably, such measurements are to be carried out in situ, e.g., in an etching environment.
Optical methods are known for this purpose. They are very sensitive and do not require a physical contact, like mechanical probes, with the sample. For measurement accuracies in the micrometer range focus methods or light sectioning methods are available. Both are based on observing light patterns that are projected onto the surface under test and change when height variations occur. In the former case, the shape of a focussed light spot is analyzed (and kept constant in a closed-loop control system) and in the latter the lateral displacement of a light spot or a light band is determined. Moire techniques project the shadow of a grating onto the surface under test which is observed through another grating. They also belong to the light sectioning methods with resolutions in the micrometer range.
For higher accuracy up to the submicron and nanometer range interferometric methods could be used where height variations of the surface lead to phase changes of a measurement beam which, after reflection at the surface, is superimposed with a reference beam to obtain an interferometric fringe pattern. Such interferometric methods are, however, limited to regularly reflecting, smooth surfaces as a rough and scattering surface will destroy the phase relation of the beams. Rough surfaces can only be analyzed interferometrically if oblique angles of incidence are used at the price of a complicated optical set-up, shadow-casting and non-applicability to roughness measurements. Another drawback of interferometric methods lies in the limited unique measuring range (in the order of half the wave length for monochromatic illumination) so that surfaces with pronounced profiles are only difficult to be measured.
Known interferometric methods use two or more interfering beams. An example of a three-beam interferometric profile measuring arrangement is described in Applied Optics, Vol. 22, No. 12, pages 1893-1897, published June 15, 1983 by K. Almarzouk. The phase of the middle beam is influenced by the thickness of an object with respect to the two symmetrical reference beams that are reflected at reference areas reserved on the sample. Apart from high reflectivity, this method, therefore, requires also a special sample preparation.